Conventionally, seat belts have an oblique portion and a ventral portion which are applied respectively with a certain tension against the chest and the pelvis of the person using the seat. When a shock on the vehicle tends to throw the user towards the front of the vehicle, for example during an accident, the seat belt is blocked, in a manner known itself, to firmly hold the user against seat.
In this case, the user is then subjected to a high deceleration and, under his or her inertia, moves in relation to the belt. This movement can cause serious injuries to the body, in particular abdominal injuries, when the user's pelvis tends to pass under the ventral portion of the seat belt by the sliding of the user's buttocks on the seat pan. This phenomenon is generally called sub-marining.
To solve this problem, it is already known, especially in French patent document FR-A-2747080, to use a bar, designated hereafter as "anti-sub-marining cross-member" or "anti-sub-marining bar", which is placed crosswise in the seat pan and which extends during a shock to limit the movement of the user's pelvis towards the front of the seat.
The cross-member is connected by its ends to two triangular-shaped links installed so as to pivot on the seat pan framework. In a rest position, the cross-member is located more or less level with the user's buttocks and, during a shock, the cross-member tilts upwards and towards the front of the seat, under the action of a belt pyrotechnical pretensioner, to an extended position, to form an obstacle preventing the user's buttocks from moving forwards thus preventing the buttocks from passing under the ventral portion of the belt.
When the cross-member is in an extended position, the user's pelvis exerts a thrust on the cross-member which may cause excessive compression of the user's lumbar region. Use of energy dissipation means is then provided to limit the reaction force transmitted by the cross-member to the user by enabling, from a predetermined threshold, progressive retraction of the cross-member into the seat pan, retaining but enabling a certain movement of the pelvis.
The energy dissipation means are, for example, oblong grooves, made along one of the sides of the triangular links, in which the ends of the cross-member move. As the width of the said grooves is appreciably lower than the transverse dimension of the ends of the cross-member, the energy dissipation is obtained by the deformation of the edges of the groove by the ends of the cross-member from a determined threshold.
In a known system, during extension, the cross-member moves both upwards and towards the front of the seat. At rest, the cross-member is located more or less at the rear of the seat at a location where the weight of the user may hinder the start of extension or at least require a relatively high force to extend the cross-member.